From nobody Mon Jun 15 19:03:37 2026 Received: from mail-pj1-f44.google.com (mail-pj1-f44.google.com [209.85.216.44]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DFD483A75B6 for ; Mon, 13 Apr 2026 08:20:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.216.44 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776068437; cv=none; b=AR1D6zs0J3mnuL8UdIqcIxlEsqpeEF+pyZvKYitbAQ8J5CLv54ddDK1taJfH9O0vV3E8Co8vOqSBLd6a9GUozPN1dOB4IvksQpjGVusSuTONOd62QeAcwmZKaayxq501iTvp9HFxtDf53EJfUzpz6hSvJjzCjmescCLrrAMxOt0= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776068437; c=relaxed/simple; bh=ZpxPLeXpDudAHB1vfPu3H3+tflsKWNVqI/+7E/D8yl4=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=rQtH/iiU7yys4S5chX+TgLPFdJsoPgF8j1kP/A/azMnbXYX/CTFXfc9u/AQ0eA7ZlqVJHm3DxkmqPFneZi0XWIOXZR0PUy7zF+6+udCRC+w/Lc89HG92LxBjhyic340Zfq1Pg5CB9WguQMJf1hHyYTBuim64quT1IciQWZItTu4= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=Ik0jwPp8; arc=none smtp.client-ip=209.85.216.44 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="Ik0jwPp8" Received: by mail-pj1-f44.google.com with SMTP id 98e67ed59e1d1-35d90833cacso2497813a91.2 for ; Mon, 13 Apr 2026 01:20:35 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20251104; t=1776068435; x=1776673235; darn=vger.kernel.org; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:from:to:cc:subject:date:message-id:reply-to; bh=kl3F5dX0VanlFuC2/COyBNs0Ns8vPwKSkzIIDi55UM8=; b=Ik0jwPp8k0y0m0xr4hPBBEGpDpz0v6AqhlIrbZHFJjRD/O6ft8PNVAId/G7vVf3lx7 764RHEjSiUwD5IIv/AwWxxS1/wuiEObyU/qlNO7k47PqFmmZNgHPKBqxx6EbgKDQ1AWh npYLBb1oGrnFJXVe8q05444EZHh1Sy+lIon31XBhetoFECZO7L2T50g9b14YJn1Fu5X2 9rG4GrqTSkwJYi5wtD5jh34NbgD4DKU4Mn0UZHerwwdHNs0tJ2KPrUNejnYlYwJ3t/uw kXyR0A8Ub1i4IJIy+IjdCzrUKdV68ggnxx9Cl0t+h6D2r4pvYHsdyYUA4F3dWhBDRH/E aiVA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1776068435; x=1776673235; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:x-gm-gg:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=kl3F5dX0VanlFuC2/COyBNs0Ns8vPwKSkzIIDi55UM8=; b=pr4nZ5YrClj/fPI9DMQ+P2zNM0hOOngxXc789eSNG3emAyIzLHPjJiN+SIKZ7tdOzl w0/nse5XG9Cp1flTrqBemnhvOPTU8iWV3g+QXPDpuSgRej8HczdsEZPztt+bLSGr6mHq mZcS6VguA/fezdK2nv6Sp/X3gdZBU6Obedrvov7+Kaqoog29CGAZTw6tMvkyglQDCsE0 bUE2ircDMKnlCFvkBMj/o00cz9JZEgq1XW2HfCZJSwJ10iQ6rBsKuopuGClae/VgPzug lZGw9lBnqnIeiT2DcDMpSjcwxR85zkcEchUtIlnM5yVRXbRxWyYjPvvJ73CsZbS2dV/h Bfbg== X-Forwarded-Encrypted: i=1; AFNElJ8sRv+M1IXHScPYHgGfigRlz8t331Cr67Tp7s+74NMl/cKoNjPRluGC3zhcONq79fDq0DQ6/3i4ixU/IQs=@vger.kernel.org X-Gm-Message-State: AOJu0Yzk47C+rq6LU7+8vKBebIO7xd023jqvKdVMfeAH70WqEPkZbKk6 9Hprwk0jmJnxrQYmnvFQ+nj45by6rhzKNzqU/QsaB6ZjBnL1NHZJg5Z5 X-Gm-Gg: AeBDieu8PSfWlBHsQvAA248ka28UDFcGgqStKb3jwJZJBhmxAYbqiEs9P20tv+RvoB5 l3dWaY3QfWoW1IsPujR9yDSz4PrzJqIo1jStw0NbMlCbbBRn62FvlsOCPcCMQgvv5OTvm0hYn95 eudVTIK3DEXjksxKylVMsYins5cP562iPt3SWnio8yPid/VB3ZMtA/1p20X4aXSplUC4SxM+Lk0 ZxqwAYNrJZsrx/1B8o6JC2DVyq6YYyTcjsyOZ9qdoVUkSak05rKjYqmIZSqB2F9vZ+5u+K8D5p6 UYWWhDDp92/caGg5I/48W9/4tiMVcgS9kklRBQOKXOaZqZNW1stgtCrSBiqsfwF/0qWSF/f2BWj DVwsEhrpyaJ6Fu6lPBk4BhrkQ3k7uedf3DmigrUMiMm77nKIAXxUywfabvFONDpJmJN+/YVgzj+ fBjx5lhkQkua8YVsVW99zxVnUONM5AfzvL X-Received: by 2002:a17:903:368c:b0:2b0:4ec8:fb92 with SMTP id d9443c01a7336-2b2d5a8ec02mr91724945ad.39.1776068435146; Mon, 13 Apr 2026 01:20:35 -0700 (PDT) Received: from localhost.localdomain ([180.167.178.215]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2b2d4f0983asm116757705ad.51.2026.04.13.01.20.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 13 Apr 2026 01:20:34 -0700 (PDT) From: "Kito Xu (veritas501)" To: jhs@mojatatu.com, jiri@resnulli.us, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com Cc: horms@kernel.org, victor@mojatatu.com, pctammela@mojatatu.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, "Kito Xu (veritas501)" Subject: [PATCH] net/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow Date: Mon, 13 Apr 2026 16:20:27 +0800 Message-ID: <20260413082027.2244884-1-hxzene@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" tcf_mirred_act() checks sched_mirred_nest against MIRRED_NEST_LIMIT (4) to prevent deep recursion. However, when the action uses blockcast (tcfm_blockid !=3D 0), the function returns at the tcf_blockcast() call BEFORE reaching the counter increment. As a result, the recursion counter never advances and the limit check is entirely bypassed. When two devices share a TC egress block with a mirred blockcast rule, a packet egressing on device A is mirrored to device B via blockcast; device B's egress TC re-enters tcf_mirred_act() via blockcast and mirrors back to A, creating an unbounded recursion loop: tcf_mirred_act -> tcf_blockcast -> tcf_mirred_to_dev -> dev_queue_xmit -> sch_handle_egress -> tcf_classify -> tcf_mirred_act -> (repeat) This recursion continues until the kernel stack overflows. The bug is reachable from an unprivileged user via unshare(CLONE_NEWUSER | CLONE_NEWNET): user namespaces grant CAP_NET_ADMIN in the new network namespace, which is sufficient to create dummy devices, attach clsact qdiscs with shared blocks, and install mirred blockcast filters. BUG: TASK stack guard page was hit at ffffc90000b7fff8 Oops: stack guard page: 0000 [#1] SMP KASAN NOPTI CPU: 2 UID: 1000 PID: 169 Comm: poc Not tainted 7.0.0-rc7-next-20260410 RIP: 0010:xas_find+0x17/0x480 Call Trace: xa_find+0x17b/0x1d0 tcf_mirred_act+0x640/0x1060 tcf_action_exec+0x400/0x530 basic_classify+0x128/0x1d0 tcf_classify+0xd83/0x1150 tc_run+0x328/0x620 __dev_queue_xmit+0x797/0x3100 tcf_mirred_to_dev+0x7b1/0xf70 tcf_mirred_act+0x68a/0x1060 [repeating ~30+ times until stack overflow] Kernel panic - not syncing: Fatal exception in interrupt Fix this by incrementing sched_mirred_nest before calling tcf_blockcast() and decrementing it on return, mirroring the non-blockcast path. This ensures subsequent recursive entries see the updated counter and are correctly limited by MIRRED_NEST_LIMIT. Fixes: 42f39036cda8 ("net/sched: act_mirred: Allow mirred to block") Signed-off-by: Kito Xu (veritas501) --- net/sched/act_mirred.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/net/sched/act_mirred.c b/net/sched/act_mirred.c index 05e0b14b5773..5928fcf3e651 100644 --- a/net/sched/act_mirred.c +++ b/net/sched/act_mirred.c @@ -444,8 +444,12 @@ TC_INDIRECT_SCOPE int tcf_mirred_act(struct sk_buff *s= kb, tcf_action_update_bstats(&m->common, skb); =20 blockid =3D READ_ONCE(m->tcfm_blockid); - if (blockid) - return tcf_blockcast(skb, m, blockid, res, retval); + if (blockid) { + xmit->sched_mirred_nest++; + retval =3D tcf_blockcast(skb, m, blockid, res, retval); + xmit->sched_mirred_nest--; + return retval; + } =20 dev =3D rcu_dereference_bh(m->tcfm_dev); if (unlikely(!dev)) { --=20 2.43.0